Slow road to reusability

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The shift to reusable launch vehicles will be far slower and more incremental than was once considered possible and desirable

Over 30 years ago, in his film 2001: a space odyssey, director Stanley Kubrick gave us his vision of a future in which man could travel from the Earth to the moon and beyond in a seamless space transportation system. As 2001 approaches, that vision is still a long way from fulfilment.

Before man had even set foot on the moon, Kubrick asked the US aerospace industry to help create his vision of the future. It stands as a testament to their far-sightedness that NASA now uses the promotion poster for 2001, showing the Pan Am Clipper aerospaceplane, to illustrate its concept for a fourth-generation reusable launch vehicle (RLV) that could enter service around 2040 and allow routine passenger space travel.

NASA now acknowledges that a truly reusable launch vehicle is still several decades away. Until recently, it was planning to replace the partially reusable Space Shuttle with Lockheed Martin's single-stage-to-orbit (SSTO) VentureStar in 2005. Now the space agency does not plan to select a commercially developed second-generation RLV until 2005, and that vehicle will augment, rather than replace, the Shuttle, at least initially.

NASA has been pursuing RLVs for several years, with the intention of substantially reducing its launch costs, which consume 25% of the agency's annual budget. The recent change in thinking is the result of an increased emphasis on safety, and the need to guarantee access to the International Space Station.

Safety comes first

In common with the commercial space industry, NASA has recognised that low launch costs cannot be achieved at the expense of launch vehicle reliability. In the commercial satellite arena, a launch failure can mean loss of revenue and higher insurance premiums. For NASA, the price of a failure could be far higher.

The result is the shift to a phased approach, under which the safety and reliability of the Shuttle will be enhanced with upgrades while NASA develops technology for second- and third-generation RLVs. This could see the Shuttle stay in service until 2025, initially augmented and progressively super-ceded by new reusable vehicles.

NASA has set goals for each new generation of RLV. The second generation, now expected to enter service around 2010, is to offer a tenfold reduction in cost and a 100-fold increase in safety over the Shuttle. This equates to a cost per kilogramme of payload delivered to low-Earth orbit of $2,200 ($1,000/lb), and a probability of losing the crew in one in 10,000 missions.

Targets for the third-generation RLV, expected to enter service after 2025, are costs 100 times lower and safety 10,000 times higher than those of the Shuttle. That means around $200/kg to orbit and a loss rate of one in 1 million - comparable to a commercial aircraft. And NASA does not stop there, envisioning a fourth-generation RLV that would provide a 1,000-fold reduction in cost and 20,000-fold increase in safety.

"By the year 2040, there will be no distinction between a commercial airliner and a commercial launch vehicle," the agency says. "Aerospace vehicles will be capable of operating in a seamless regime from the Earth's surface all the way to orbit." Finally, Kubrick's vision could be achieved.

But that goal must seem a long way off for an agency that has yet to demonstrate the technology for a second-generation RLV. It has three such programmes under way: the Lockheed Martin X-33, a subscale, suborbital prototype of the SSTO VentureStar; the air-launched Orbital Sciences X-34, and the Boeing X-37, the potential basis of a reusable upper stage for a two-stage-to-orbit (TSTO) launch vehicle.

Other demonstrators are planned en route to NASA's 2005 RLV selection deadline, and could include flight tests of a rocket-based combined cycle engine, seen as the most likely powerplant for a third-generation vehicle (see P118-120).

Likely candidates for the second-generation RLV are a Shuttle derivative, the VentureStar and a new TSTO vehicle, possible using the USAir Force's Evolved Expendable Launch Vehicle as the first stage and an unmanned crew/-cargo transfer vehicle as the upper stage. The latter would bear some resemblance to Europe's defunct plan to launch the Hermes unpowered reusable spaceplane atop the expendable Ariane 5.

After many false starts, Europe's hopes for developing a fully reusable launch vehicle stand at a crossroads. At the time of writing, a final decision on the European Space Agency's (ESA) Future Launchers Technology Programme (FLTP) was expected. If approved, this three-year, c60 million ($60 million) programme will provide the launch pad for a European effort on a next generation launch vehicle that is intended to cut dramatically the cost of access to space.

Europe is only too aware it that lacks a fully funded RLV programme and that the USA is significantly ahead not only in technology proven on the Shuttle, but also through its extensive ongoing demonstrations. "The year 2000 is one in which we will see a great deal happening in the USA," says French space agency CNES. "Europe must make a significant effort if it is to catch up."

France has gone further than any other country in Europe in studying the technology of reusable space vehicles. During the early 1990s, it was deeply involved in the Hermes programme. But this was abandoned on cost grounds, although considerable work had been carried out on technology issues and design problems.

Since then, the Apollo capsule-like ARD atmospheric re-entry vehicle, for which Aerospatiale Matra was project leader, has been launched by an Ariane 5. Final results are still being analysed, but the project has yielded considerable information - and revealed significant gaps in Europe's ability to predict the extreme aerothermal and aerodynamic conditions encountered during re-entry.

The FLTP is a follow-on to several previous European RLV studies, all of which were short-lived. These included the German Saenger two-reusable-stage launcher and the British Hotol SSTO vehicle, both of which were abandoned as overambitious. ESA's Future European Space Transport Investigation Programme was more successful, studying several RLV concepts as well as the cost and operational aspects of reusable space vehicles.

European work

While the size of the planned FLTP is minuscule in comparison to efforts under way in the USA, it will draw on work carried out so far and will take Europe to the point where it will be able to decide the configuration of an RLV and ensure potential technology pitfalls are understood.

"We have talked about this kind of vehicle for a long time, but there is still a huge amount of work to get the economic and operational aspects right," says Aerospatiale Matra executive vice-president, space programmes, Bernard Humbert.

He reflects mainstream opinion in Europe that the Ariane 5 and its derivatives will continue to be the most cost-effective way of providing access to orbit for the next 10-20 years at least. "We do not see any great urgency to move rapidly to develop a reusable launcher," he says. But he adds that if any of the RLV programmes under way in the USA took root and became a major programme, "we would definitely have to react".

In its RLV roadmap, Aerospatiale Matra suggests three concepts, each representing a step up in technological challenge: single and fully reusable launch vehicles (SRLV and FRLV) and an SSTO. The two-stage SRLV would be launched with a single reusable element - probably the booster, "since it is much more difficult to bring an orbital vehicle back to Earth", says Humbert. The reusable element would either return to the take-off point or to another base up to 4,000km distant.

The challenge, says Humbert, is not to achieve some arbitrary step-change in launcher performance, but rather to "move towards a higher performance system in the hope that we will arrive at a reusable system that is less expensive to operate than today's expendables". He cautions there is a "huge amount of work to ensure we get the economic and operational aspects right".

One clear result of Aerospatiale Matra's studies is that to be economic an RLV must be capable of at least 20 launches a year (NASA has set a goal of 100 launches a year for its second-generation RLV, and 2,000 a year for the third generation). Technologically, Europe and the USA face the same challenge - how to achieve a major reduction in vehicle weight and increase in engine performance so that launch costs can be reduced dramatically.

Because most of Europe's limited funding for launch vehicle research is directed at Ariane 5 developments, little serious work has been carried out on RLV technologies. "We have a long way to go", says Humbert. "But we must start now or we will be left behind."